Filter material



Oct. 21, 1952 s, KATZ 7 2,614,655

I FILTER MATERIAL Filed Aug. 10, 1945 AIR imassuns Patented Oct. 21, 1952 FILTER MATERIAL Sidney H. Katz, Edgewood Arsenal, Md., vassignmto the United States of America. as represented by the Secretary of War Application August 10, 1943, Serial No. 498,062

I (c1. 1s3 44) (Granted under the act of March 3, 1883, as

2 Claims amended April 30, 1928; 370 0. G. 757) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon.

This invention relates, generally, to fiber mediuins'for use in connection with gas masks and the like, and it has particular relation to filter mediums impregnated with finely divided, dry asbestos fibers so as to have great resistance to deterioration by oil or liquid smokes or fogs, and high filtering efficiency with relatively low airresistance.

In general, the canisters of gas masks and collective protectors used for protection against chemical warfare agents, contain two types of airpur-ifying mediums. One of these mediums is the so-called mechanical filter or filter which is adapted to remove exceedingly small solid and liquid smoke particles floatin in the air such as dip'henylaminechlorarsine, diphenylchlorarsine,

diphenylcyanarsine, and chloracetophenone. The other purifying medium usually comprises a mixture of activated charcoal granules andsoda lime or other granular absorbents, and has the function of removing toxic vapor molecules from the air by adsorption and chemical reaction, such as, phosgene, chlorpicrin, mustard gas, lewisite, and chloracetophenone. The mechanical filter is, generally, in the form of a pad or multilayer web of fibers held together in any convenient form and so arranged that the inspired air passes therethrough, wherein the irritant smoke particles are caught and retained in the fibrous meshes, allowing the air with the toxic vapors to pass on through to the charcoal and soda lime wherein the final stages of purification in the canister take place.

The mechanical filters have been made from a variety of fibrous materials such as felt, cotton pads, water-laid paper webs, air-laid webs, and fabrics. Prior to this invention, the best all-. around filter material was a water-laid paper web of the type commonly known as filter paper, which had been impregnated with minute carbon particles. More specifically, the standard filter material in use prior to this invention, and which was considered the best available, was a filter paper web having an alpha cellulose content of 90% ormore, a basis weight of about 30 to 60 pounds, a web thickness of about 0.012 to 0.030

, to be at all promising and suitable.

V of an inch, and an air resistance of about 5 to 12 mm. of water as described hereinafter, which filter paper web had been impregnated with minute carbon particles by drawing the s'mokefrom burning acetylene through the paper. The mechanical filter consisted of a plurality, of layers of this paper web.

However, with the development of oil smoke for large scale screening operations, that is, smoke generated by the vaporization of suitablegrades' of petroleum, it was "found that suflicient concentrations of this type of smoke were set up to cause the rapid deterioration of the standard filter elements used in the canisters thereby permitting subsequent penetration by excessive amounts of toxic smokes, which it was the primary function of these filter elements to remove from the inspired air. On investigation, it was found that the deteriorating effect of the oil screening smokes was due primarily to their action on the fine particles of carbon impregnated into the paper. Filters made of unimpregnated alpha' Web were testedand found to be affected to little or no extent by the oil screening smokes. Of course, the unimpregnated alpha webs cannot be used as suc'hsince they do not afford sufii-cient protection against -toxic smoke without excessive air resistance. Accordingly, it becameurg'ently necessary to discover some material othe than carbon with which to impregnate the mechanical filter, which material would not be subject to deterioration by the oil smokes, and which would impart highfiltering efilciency with relatively low air resistance 1 Of' the many different materials which might conceivably be used to replace carbon, such as, glass wool; calcium silicate, pumice, talc, ester gum, rosin and asbestos, only asbestos was found It was first attempted to incorporate the asbestos by mixing a water suspension thereof with the cellulose fiber pulp, and'feed this stock to a paper making machine. However, diificulty was experienced in practicing this method because of the loss of a considerable amount of the most desirable finely divided, asbestos fi bers which were not retained in the water-laid web, but which passed on with the water leaving the cylinder, and because the resulting asbestos-bearing paper lacked good filtering eiiiciency, andhad a relatively high air resistance. Furthermore, it Was difficult tomake a uniform grade of asbestos-bearing paper by this method on a quantity production basis. This type of paper made by blending asbestos and 'cel lulose pulp into the stock is referred to as asbestos bearing paper.

It was conceived according to the present invention that perhaps the asbestos could be satisfactorily incorporated into a filter paper web in the form of air-floated, dry asbestos fibers. The great mobility of the asbestos fiber-s suspended in the air would tend to equalize the porosity of the paper web, and the finest fibers, most desirable for forming the filters, would be mostly retained in the filter paper. On experimentation, it was found that this new method was satisfactory and that a filter material havin very desirable properties could be manufactured thereby, the details of which are described hereinafter.

Accordingly, the primary object of this invention is the provision of an improved filter medium for use in connection with gas masks and the like, adapted to manufacture on a quantity production basis, and characterized by its great resistance to deterioration by oil smokes, and its high filtering efficiency for toxic and irritant, solid and liquid smoke particles with relatively low air resistance.

A further, and very important object of this invention is the provision of a high grade mechanical filter material for gas masks and the like, comprising a fibrous filter support impregnated with finely divided, dry asbestos.

A more specific object of this invention is the provision of an improved filter for gas masks and the like, comprising a high alpha content, filter paper web impregnated with air-floated, finely divided, dry asbestos fibers,

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.

For a more complete understanding of the nature and scope of this invention, reference may be had to the following detailed description thereof setting forth by way of illustration certain presently preferred specific embodiments, taken in connection with the single drawing which diagrammatically shows one type of machine in which asbestos impregnated filter material may be made according to this invention.

Several dilferent fibrous materials may be used into which the finely divided, dry asbestos fibers may be impregnated, such as filter paper, felt, and air-laid webs of cotton linters. Although felt has a relatively low air resistance with good filtering efficiency, it is not entirely satisfactory from a mechanical standpoint since it does not withstand well the normal stresses encountered in the impregnating process and in the canister manufacture, without stretching out of shape. However, in certain types of canister designs this mechanical weakness of felt would not be an important factor. A fibrous web formed from air-laid cotton linters may also be impregnated with dry asbestos fibers to form a filter having desirable properties. However, the presently preferred base material is a high alpha cellulose content, filter paper web of the type previously used for impregnation with acetylene smoke, but preferably having a lower air resistance.

The most suitable type of filter paper to date appears to be one having a fuzzy surface, a soft texture, an alpha cellulose content of 90% or greater, a basis weight of 30 to 60 pounds, a web thickness of from about 0.012 to about 0.030 of an inch, and an air resistance of from about 3.6 to about 18 mm. of water as described hereinafter. Preferably the filter paper should'be one the air resistance of which is in the lower part of the range permitted. For example an air resistance of around 4 mm. of water is very satisfactory. It has been found that even tiny holes in the low air resistance filter paper may be tolerated since impregnation with the dry asbestos fibers mends or fills up such tiny holes. The fuzzy surface on the paper is needed so that the asbestos will penetrate and adhere to it.

In determining the air resistance of individual layers or sheets of filter paper, air is passed through a 100 sq. cm. sample of the paper at a rate of fiow of 85 liters per minute, and the pressure drop is measured with a manometer in millimeters of water.

Prior to this invention it had been found that filter paper of the type used for the mechanical filters of gas masks, could be satisfactorily waterproofed with methylsilicon chlorides. This waterproofing was carried out by exposing the filter materials to be waterproofed to a vaporair mixture of methylsilicon' chlorides, and after a time-lapse of about 24 hous, neutralizing the acid formed by treatment with ammonia vapor. The acid formed by treatment with the methylsilicon chlorides progressively weakens the filter materials. Neutralization of the acid with ammoniaserves to halt and prevent further weakening of the paper. However, a greater waterproofin efiect is obtained if the neutralization with ammonia is deferred to the latest possible time without resulting in weakening the filter material below a predetermined minimum.

Although it was not thought that the waterproofing treatment would have any effect on, or connection with, the filtering quality of the asbestos impregnated material, froma number of tests conducted, there appears to be a definite and unexpected indication that for some reason or other the waterproofing treatment improves the filtering quality of the asbestos impregnated material.

The following table gives information on the different types of filter paper which may be used:

Table I Web Air Resist- Type of Paper Manufacturer g g gg inch Water Knowlton Bros. 0.023 13.8 to 18 0. 023 13. 8 to 18 O. 020 6. 9 O. 013 3. 8 0. 019 3. 8 0. 019 3. 8 0. 022 6. 5 0. 022 6. 5 0. 022 7. 0 0. 020 3. 6 O. 020 3. 6 O. 020 3. 7 0. 018 4. 2 D0 0. 022 6. 8 Alpha Web. i 0. 020 6. 7 Asbestos bearing 0.020 16.0 to 18. 0

Several different grades or types of asbestos have been used for the impregnation of filter materials. Table II contains a list of the various types and grades of asbestos which have been used, indicating the source from which obtained, and giving certain other pertinent information.

- missible range in dimensionsof the asbestos is difiicult to obtain.

Tablell' Manufacf Source.f1'.om.wh1ch Type oi Asbestos obtained, tlgglg Remarks MricanBlueLnu ,s Johns Manville..' #1206 Airrseparated for cleaning;v VermontAsbestosFloat The Ruberoid Go Very few fibers or rod V L shaped particles. I Medium vFiber Acid-washed Fisher Scientific Co. and, Gooch e Amphibole. Eimer and Amend. I

- Maryland Acid-washed Mi- Powhatan Mining C'o Gooch First, part 0f:.m1cron1ze1t cronized. run. a

Dow; .do. Gooch Latter part of micronrzer 5 run. morev representative G h of continuous operation. Maryland Acid-washed .do 00c I -C l JohnsManvilla; 4M12 Cleaned washed andshoit- Quebec hrysotl e ened in Spr0ut.-Waldron 4M12 cutter bec Ch I sotileMicronized. do llg yland ude Powhatan Mining Co. Contains l0%. iron. crude 1 matetlal for G'oochr Grade.

South American Blue d0 BB 5% dlrt .removed by dry.

' screening. 7 Crude Canadian Chrysotile- Arthur D. Little, I'nc. }Not'washed.

Of the various different types. of asbestos used,

it appearsthat the Canadian Chrysotile asbestos is the best, that the African Bluevariety is seclengthsmany times their diameters, for example,

from 50 to 100. microns, appear to provide. one very satisfactory grade. The. dimension characteristics of the asbestos fibers.will', of course-to satistactorily is the so-called Mikro Pulverizer,

Bantam model, Serial No. 6018, manufactured by the:Pulverizer Machinery 00;, Roselle Park, N. J.

This pulverizer is a small (5 in. rotor diameter),

high-speed (upto 16,000 R. P. M.) hammer mill. The-asbestos to. be pulverized is fed by-a screw conveyorinto the; working area of the apparatus. The.pulverized-material passes out through a screen. (screen l3) at thebottom and into a discharge pipe. The screen at the discharge is replaceable; permitting the use of various screen a large extent dependupon the particular defibering'or pulverizing 'apparatus used for disintegration. It has been found that although the asbestos fibers can be reduced to as small a diameter as possible with a general improvement, infiltering efiiciency, as the diameter is reduced, the lengthsof the fibers should not be shortened too greatly. Therev are indications that if thelengths' of the asbestos fibers are shortened too greatly. when the asbestos is used to impregnate the filter material, the air resistance is increased disproportionately to the filtering value.

Because of the minute dimensions involved, complete and accurate information as to the perfibers Reference may now be had to the drawing 1 which shows apparatus by which filter paper of the type described may be impregnated with airfloated, dry asbestos fibers, on a quantity proreverse direction from that in which anovershot sizesfor handling difierent materials.

In. certain instances difiiculty was encountered because of agglomeration of the asbestos and the plugging of the discharge screen [3:1 It was found that such agglomeration, and plugging of the screen could be eliminated by adding a small amount (about 1%) of charcoalto the asbestos as it. was fed to the pulverizer l2. However, it was noted (1) that the amount-of charcoal should be kept at aminimum because of its tendency'to increase the smoke penetration of the paper and (2) that. the minimum amount required for obtaining the'desired effect varied with the weather conditions, morebeing needed on days when high humidity prevailed.

From the pulverizing equipment, the pulverized asbestos is fed into a. rectangular box-shaped top section E5 of an impregnating chamber indicated generally at Hi. In one embodiment of the apparatus, the top section l5 was 24. inches long and wide enough to accommodate a web of paper 8 inches in width. The bottom section l! of the impregnating. chamber [6 is in the form of an inverted. rectangular pyramid, the upper end of which has substantially the samev dimensions as water wheel normally revolves, 'so that. the" buckets serve to scrape and-push the asbestos through an adjustable slot 9 located in thebottom of the hopper 6. The width of the" slot or opening 9 may be adjusted by, setting a closure member 10 in different positions.

From the hopper 6, the asbestos is delivered to a pulverizer l2 provided withpan outlet screen indicated diagrammatically at. 13. One. type of I pulverizer which hasbe enlfound to serve very the rectangular top section 15. .The upper end of the lower section I! is covered with a heavy G-mesh wire screen I 8, and the lower, smaller end of "the pyramidsection I1 is attached to a vacuum pump 20.. Thelowersection H .and the vacuum pump 210{shouldbeso designed that airmay be a linearvelocity' 23 extend from the header. pipe 2| as shown.

I These distributor pipes 23 are provided with air holes 24 in the undersidesv thereof, the number of which progressively increase in concentration to-- ward the ends of the distributor I pipes 23'jaway from the header pipe 2 l.

This unequal concentration of the airholes 24 compensates forthe decrease in air pressure at the far ends of the distribution pipes 23.

The filter paper to be impregnated is provided on a roll 25 on one side of the impregnating chamber l6, and the paper web 28 extends through the impregnating chamber 18 and is wound up on a roll 21 on the opposite side thereof. roll 28 aids in the smooth feeding of the filter paper web 25 into the impregnating chamber IS. The roll 21 is driven by friction engagement with a roll 3%, driven by a constant speed electric motor 3!. The strength of the filter paper sheet 26 is sufiicient so that it may be unwound without breaking from the roll 25 by the tension supplied by the drive roll as at a paper speed of about 175 feet per minute.

As the pulverized asbestos enters the top section of the impregnating chamber It, the turbulent currents of air therein serve to evenly distribute and suspend it throughout the upper section H5. The air jets from the outlet holes 24 serve to drive and force the individual asbestos fibers-into the filter paper web 26 as it passes through the chamber 15, and to blow off that portion notfirmly attached to the paper causing it to recirculate. The top section l5.is provided with an exhaust vent 32 which is connected to thesuction side of a blower 33, as shown, The blower 33 serves to create a slight negative pressure or vacuum in the top section thereby causing a small amount of air to leak into the impregnatingchamber It at the slots where the paper web 26 enters and leaves. This inward leakage of the air prevents the loss of asbestos through the slots. The vacuum pump 2b also helps to create the slight vacuum or negative pressure in the chamber 16.

The air resistance of the paper sheet leaving the impregnating chamber i6 may becontinuously measured by apparatus shown diagrammatically at 35. The resistance indicator 3 3 comprises an inverted rectangular pyramid having a 100 sq. cm. base covered with a coarse wire screen and fitting against the underside of the sheet 20. Air is drawn through the apparatus 32 at a rate or" 85 liters per minute,,and the vacuum inside the pyramid measured by a 'Merian (inclined tube) draft gage designed for very accurate reading in the. resistance range normally used. The air resistance of the impregnated sheet 28 should be uniform and serves as an indication that proper impregnation is being obtained. 7

Although no specific apparatus is shown therefor in the drawing, the asbestos as received at the plant is usually so coarse that it is preferably pretreated before delivery to the hopper This may-be satisfactorily done by passing it through a first run in a pulverizer similar in shape to pulverizer 52. A Wiley Mill No. made by Arthur H. Thomas, Inc. has also been found to be satisfactory for the preliminary treatment.

An idler This mill was driven by a 1 /2 H. P. motor, belted down to 800 R. P. M. The setting onknife clearances and choice of screen mesh are varied, depending on the asbestos used. A /3 inch knife clearance and a 0.078 inch perforated screen gave good results. Gther fiber separating equipment such as ballmills, and coiiee mills may also be used.

As the degree to which the paper web 26 is impregnated increases, the air resistance thereof increases proportionately. It has been found, however, that for each type of asbestos fibers there is an optimum resistance'to which the paper shouldbe impregnated. That is, the filtering efiiciency is improved as the degree of impregnation increases until an optimum is reached where there is very little increase in filtering eificiency with a disproportionate increase in air resistance. Generally speaking, the impregnation of a paper web having an initial air resistance of 4 mm. of water, to an air resistance of between 10 and 16 mm. of water, will give a superior filter for smokes. I

It will be understood that the apparatus and arrangements shown in the drawing are illustrative of one type of impregnating apparatus which has been found to be satisfactory, but thatother types of apparatus may be used.

Instead of drawing air-floated asbestos fibers through a filter paper web, as described above in connection with the drawing, finely divided asbestos fibers may be evenly sifted onto the paper web and then brushed as air is drawn through the web. Although the test results on individual sheets prepared in this manner were relatively poor, when multilayer filters were made from the sheets the test .results therefor were reasonably good. v v

The asbestos impregnated filter paper made according to this invention has been found to equal and in most instances to exceed all of the standard requirements contained in the rigid specifications established by the Chemical Warfare Service of 'theArmy, which'is the agency responsible for this material. new type of filter material has unusual resistance to deterioration caused by screening smokes of the oil type.

Since certain further changes may be made in the particular materials employed and procedures used, Withoutdeparting from the scope of the invention, it is intended that all matter described above, or shown in the accompanying drawing, shall be interpreted as illustrative and not in a limiting sense. I

I claim: l. A filtering medium comprising, paper having dispersed thereon and in the pores thereof by air filtering a finely pulverized'asbestos particle filtrate, said paper comprising a soft fuzzy and porous weoof cellulose fibers, said particles being-frictionally held iii-said fibers in said web. I

2. The medium of ciaim ly in which said.

asbestos particle filtrate contains an antiagglomcrating agent consisting of pulverized charcoal and constituting by weight about 1% of said particles.

STDNEY H. KATZ.

Ruminations siren The following references are of record in the fileof this patent: V

UNITED STATES PATENTS Number Name Date 321,9 5 Breyer July 14, 1885 1,349,112 'Vfeiss Aug; 10, 1920 1,798,16 Kuhn et a1. Mar. 3l, 1931. 2,061,570 Frolioh et al Nov. 24, 1936. 2,063,990 Dym Deals, 1936 2,306,222 Patnode' Dec.,22,,19;l2 2,372,437 Lathrop -et a1. 'Mar. 27,1945

FOREIGN PATENTS Number. Country Date 7 19,172 GreatBritain Aug. 26, 1911 851,577 France Oct. 2, 1939 In addition, this 

1. A FILTERING MEDIUM COMPRISING. A FILTER PAPER HAVING DISPENSED THEREON AND IN THE PORES THEREOF BY AIR FILTERING A FINELY PULVERIZED ASBESTOS PARTICLE FILTRATE, SAID PAPER COMPRISES A SOFT FUZZY AND POROUS WEB OF CELLULOSE FIBERS, SAID PARTICLES BEING FRICTIONALLY HELD IN SAID FIBERS IN SAID WEB. 